The present invention relates to a method for the production of hard metal tools or components using the powder injection molding or extrusion method.
Hard metals are composites consisting of small (μm-scale) grains of at least one hard phase in a binder phase. In the case of tungsten carbide based hard metals, these materials always contain the hard phase tungsten carbide (WC). In addition, other metal carbides with the general composition (Ti,Nb,Ta,W)C may also be included, as well as metal carbonitrides, e.g., Ti(C,N). The binder phase composition may be Co, Ni, and Fe, or combinations thereof. Co is preferred.
Titanium carbonitride based hard metals do not contain the tungsten carbide phase, i.e. WC.
Industrial production of hard metals often includes blending of given proportions of powders of raw materials and additives in the wet state using a milling liquid. This liquid is often an alcohol, e.g., ethanol, or water, or a mixture thereof. The mixture is then milled into a homogeneous slurry. The wet milling operation is made with the purpose of deagglomeration and mixing the raw materials intimately. Individual raw material grains are also disintegrated to some extent. The obtained slurry is then dried and granulated, e.g. by means of a spray drier. The granulate thus obtained may then be used in uniaxial pressing of green bodies or for extrusion or injection molding.
Injection molding is common in the plastics industry, where material containing thermoplastics or thermosetting polymers are heated and forced into a mold with the desired shape. The method is often referred to as Powder Injection Molding (PIM) when used in powder technology. It is preferably used for parts with complex geometry.
In powder injection molding, four consecutive steps are applied:
1. Mixing of the granulated powder with a binder system. The binder system acts as a vehicle for the powder and constitutes 25-55 volume % of the resulting material, often referred to as the feedstock. The exact concentration is dependent on the desired process properties during molding. The mixing is made with the binder system in molten state. The resulting feedstock is obtained as pellets of approximate size 4×4 mm.
2. Injection molding is performed using the mixed feedstock. The material is heated to 100-240° C. and then forced into a cavity with the desired shape. The thus obtained part is cooled and then removed from the cavity.
3. Removing the binder from the obtained part. The removal can be obtained by wet extraction of the parts and/or by heating in a furnace with a suitable atmosphere. This step is often referred to as the debinding step.
4. Sintering of the parts. Common sintering procedures for hard metals are applied.
Extrusion of the feedstock comprises steps 1,3 and 4 above. Instead of forcing the feedstock into a cavity of the desired shape, the feedstock is continuously forced through a die with the desired cross section.
EP-A-963454 (WO 98/18973)discloses a method of making a sintered body comprising one or more hard constituents in a binder phase by injection molding technique comprising wet milling of a powder mixture containing powders forming the hard constituents and binder phase, drying said milled powder mixture, mixing said powder mixture with organic binder, waxes and surfactant into a feedstock, molding the feedstock into bodies of desired shape in a conventional plastic injection molding machine, debinding said body and sintering. If the surfactant is introduced already during the milling operation, the level of porosity in the body is significantly reduced.